Alumina base ceramic sintered body and its manufacturing method

ABSTRACT

PCT No. PCT/JP98/04727 Sec. 371 Date Sep. 9, 1999 Sec. 102(e) Date Sep. 9, 1999 PCT Filed Oct. 19, 1998 PCT Pub. No. WO99/21806 PCT Pub. Date May 6, 1999This invention provides an alumina base ceramic sintered body excellent in wear resistance with high hardness, also with high strength and high toughness, and particularly suitable for a cutting tool or wear resistant member. This sintered body contains 5-70 % by volume of WC and 5-70% by volume of Ti (C,N) solid solution having a C/N mole ratio ranging from 1:9 to 9:1, the WC and Ti (C,N) having particle sizes of 5  mu m or less, respectively.

TECHNICAL FIELD

This invention relates to alumina base ceramics excellent in wearresistance with high hardness, high strength and high toughness, andparticularly suitable for a cutting tool or a wear resistant member.

BACKGROUND OF TECHNOLOGY

Such alumina base ceramics have been used as a cutting tool or a wearresistant member for high-speed machining and hot working where aconventional tool material such as a tool steel, a high-speed steel, acemented carbide or the like is difficult to use, because of itscomparable easiness for producing a compact sintered body amongconventional ceramic materials.

However, the alumina base ceramics is disadvantaously limited in usebecause of their poor strength and toughness.

Sintered Al₂ O₃ dispersion-strengthened with carbide or carbo-nitrideparticles such as TiC or Ti (C,N) in order to improve the properties,so-called black ceramics have been developed. However, such blackceramics is lack in reliability when applied for machining under a heavyload such as rough cutting of steel or cast iron, because of itsinsufficient strength and toughness.

Further, since a sintered Al₂ O₃ added with WC particles is improvedsignificantly in strength and toughness, and has a high hardness, it hasbeen tried to apply to wear resistant members.

For example, Japanese Patent Laid-Open No. 3-290355 discloses that ahigh hardness and high toughness sintered body having a density of morethan 90% of the theoretical can be provided by subjecting a startingpowder consisting of Al₂ O₃ added with 10-90% by volume of WC to normalsinterinig, hot pressing, or hot isostatic pressing.

It is also disclosed in Japanese Patent Laid-Open No. 5-279121 that asintered body obtained by adding 5-95 wt % of WC to Al₂ O₃ followed bysintering at 1400-1950° C., and making W₂ C phase appear due toexistence of oxygen of 0.05-6 wt. % in the WC starting powder isexcellent in strength and toughness.

However, these Al₂ O₃ -WC ceramics can not exhibit sufficient wearresistance under a severe condition of high-speed cutting of steel orcast iron, since the oxidation resistance at high temperature isinsufficient although it is surely excellent in strength and toughness.Therefore, these ceramics are hardly used for a member requiringhigh-temperature wear resistance such as cutting tool.

DISCLOSURE OF INVENTION

The purpose of this invention is to provide alumina base ceramicsremarkably improved in strength and toughness while preserving excellentoxidation resistance and wear resistance during machining under a severecondition.

This invention provides a dense sintered body of Al₂ O₃ which contains5-70% by volume of WC and 5-70% by volume of Ti (C,N) solid solution,and this sintered body exhibits high strength, high toughness, andexcellent wear resistance even at a high temperature.

Alumina powder having an α-crystalline phase of 90 vol. % or more and anaverage particle size of 5 μm or less is preferably used.

The strength and toughness are remarkably improved by adding WCparticles to the Al₂ O₃ sintered body. This improvement is resulted fromthe dispersion strengthening by highly hard WC particles and thetoughening by the action of the residual stress due to the difference inthermal expansion coefficient between Al₂ O₃ particles and WC particlescaused when the temperature lowers from the sintering temperature toroom temperature between the two kinds of grains. The addition of WC toAl₂ O₃ has an effect of suppressing the grain growth in the sintering,whereby the fine grains are maintained to strengthen the sintered body.

The content of WC is determined within the range of 5-70% by volume.When the content of WC is less than 5% by volume, the improvement instrength and toughness of the sintered body is little because of smallamount of WC phase. But, when it exceeds 70% by volume, the oxidationresistance at high temperature tends to deteriorate in addition to thedifficulty of providing a dense sintered body. However, a dense sinteredbody can be provided even with a WC addition of up to 70% by volume bysubjecting a sintered body having a relative density of 95% or moreformed by hot pressing to HIP (hot isostatic pressing) treatment.Although the improvement in strength and toughness depends on the volumefraction of WC, the sintering is difficult when the addition of WCinferior in self-sintering ability is 70% by volume or more, so that adense sintered body can not be provided.

Although the strength and toughness are remarkably improved by theaddition of WC to Al₂ O₃, compared with a mono-phase Al₂ O₃ sinteredbody, to provide a sinteled body resistant to impact, the excellent wearresistance possessed by Al₂ O₃ is lost and, particularly, the wearresistance at high temperature is significantly deteriorated. However,by incorporation of Ti (C,N) into Al₂ O₃, improvement of the strengthand thermal shock resistance is brought out by dispersion strengthening.Further, the wear due to the oxidation or the reaction with a countermaterial at high temperature is also small. Thus the high wearresistance originally possessed by Al₂ O₃ can be kept. The mole ratio ofC:N in Ti (C,N) is particularly effective within the range of 1:9-9:1.

According to this invention, the Al₂ O₃ base ceramic sintered body isable to reveal excellent wear resistance together with high strength andhigh toughness by simultaneously including prescribed quantities of bothof WC and Ti (C,N).

Sintering agents such as MgO, Y₂ O₃, CaO, ZrO₂, and the like havegenerally been used in sintering of Alumina base ceramics to promote thesintering. Although it is facilitated to sinter to dense material byusing these agents, oxide agent phases existing independently orcompound phases formed by the reaction between Al₂ O₃ and oxide agentsare grown to deteriorate the strength, the hardness, and the wearresistance under severe machining condition.

In the Al₂ O₃ base ceramics of this invention, since no sintering agentis used. It is one of features that no deterioration in variousmechanical properties resulted from those agents. Thus the Al₂ O₃ basedceramics of this invention basically consist of only three phases of Al₂O₃, WC and Ti (C,N).

Free carbon in the starting powder exists as a foreign matter of severalμm to several tens μm size in the slintered body and causes a reductionin bending strength and, in its turn a chipping or breaking off of thecutting tool during machining. In order to reduce influence of freecarbon, it is allowed to add W, Ti, or the like up to 1.0 vol. % whichhas high affinity with carbon and gives no deterioration to the sinteredmaterial. In this case, the addition of W, Ti, or the like takes placethe reaction between these additives and carbon to make WC, TiC or thelike. Thus the residual free carbon is eliminated by these reactionsduring the sintering to contribute to the improvement of reliability.

The Al₂ O₃ base ceramics of this invention can be produced bysufficiently mixing the starting powder mixture having the definedcopstion, and hot pressing the resulting mixture at 1600-1900° C. for0.5-5 hours in an inert gas atmosphere at a pressure of 50-300 kgf/cm².

It may also be produced by adding a slight quantity of a binder to thepowders, molding by use of metal mold or cold isostatic pressing or theboth, and then sintering at 1600-1900° C. for 0.5-5 hours in an inertgas atmosphere and, as occasion of demands, further performing a hotisostatic pressing at 1400-1700° C. in an inert gas atmosphere at apressure of 500-2000 atm.

MOST PREFERRED EXAMPLE OF INVENTION

Table 1 shows the relationship of tested compositions with resultedvarious properties. In the table, the sample number of No. 1 to No. 14are examples of this invention having a composition within the rangedefined. No. 15 to No. 29 are for the comparison, in which No. 15 to No.25 are out of the range of composition defined by this invention, andNo. 26 to No. 29 show examples of conventional ceramic cutting tools.

The respective samples were prepared by weighting and putting aprescribed quantities of a starting powder having average particle sizesof 2 μm or less into a ball mill, mixing those with a methanol for 20hours followed by drying and hot pressing the resulting prepared powderin a carbon die at a temperature of 1700° C. for 60 minutes at apressure of 20-25 MPa for 60 minutes. The resulting sintered body wascut and ground into bending test pieces of 3×4×40 mm and cutting toolinserts according to the standard of JIS SNGN 432, which are subjectedto various tests.

In the table, the wear resistance test was performed by use of workmaterial of FCD 450 of Φ 250×500 mm with a cutting speed of 200 m/min, adepth of cut of 1.5 mm, and a feed of 0.2 mm/rev, and the time forming aflank wear of 0.5 mm was set as life time, whereby the wear resistancewas evaluated.

The impact resistance test for examining the resistance for chipping orbreaking off of a tool was performed by use of a work material of Φ250×500 mm with four notches to give impact while constantly keeping acutting speed of 200 m/min and a depth of cut of 1.5 mm, and changingthe feed.

In the cutting test for a chilled cast steel of Φ 400×500 mm ,the wearresistance test was performed with a cutting speed of 70 m/min, a depthof cut of 1.5 mm, and a feed of 0.2 mm/rev, and evaluated by comparingthe time forming a flank weal of 1.0 mm with the life time ofconventional cutting tools.

The followings were clarified from the results of these tests.

Under the condition of so-called dry cutting using no coolant for bothcooling and lubrication in machining, wear resistance is deterioratedbecause of the low oxidation resistance of WC. The deterioration of wearresistance is proportional to the added volume of WC, and theperformance as tool is exhibited with 40 vol % or less and, preferably,30 vol % or less of WC. However, under a wet condition using coolant,oxidation of WC is suppressed because the temperature at the cuttingedge is not raised so high, and a significant improvement in the impactresistance or chipping resistance resulted from the enhanced grainfall-off resistance due to the toughening effect was rather recognized.

Further, the wear resistance effect by Ti (C,N) in addition to thetoughening effect by WC was performed in both WC and Ti (C,N)incorporated alumina to lead a long time cutting of the materialdifficult to cut in high efficient machining impossible for theconventional cutting tools.

The effect begins to exhibit with a total addition of WC and Ti (C,N) of40% by volume or more, and becomes remarkable with 50-80% by volume.With less than 50% by volume, the volume ratio of alumina is high, whichleads to a reduction in toughness, and with above 80% by volume, thesintering becomes difficult, and the hardness, the strength and thetoughness are reduced by the presence of pores in the sintered body,resulting in the deterioration of the performance as a cutting tool.

Thus, when a sintered body consists of 5-70% by volume of WC, 5-70% byvolume of Ti (C,N), and the remainder consisting of alumina having anα-phase ratio of 90% by volume or more and inevitable impurities, amaterial combining excellent wear resistance and improved toughness canbe provided.

In the composition according to this invention, the bonding strengthbetween crystal grains is enhanced by the effect of WC, the wearresistance is also improved by Ti (C,N) to reduce the adhesive wear, andmaterials difficult, to cut can be machined with a high efficiency asshown in the examples of chilled cast steel cutting.

As seen in the table, although a desired performance could beestablished in a range of TiC:TiN ratio of Ti (C,N) from 1:9 to 9:1, theeffect of addition of TiN was revealed when the content of TiN was toomuch as No. 15 and an early chipping or breaking-off by its low hardnesswas recognized. When the content of TiC was too much as No. 16, theaddition effect of TiC was revealed to slightly deteriorate thesintering performance, so that pores were apt to be left, and a promotedearly wear was caused on chilled cast steel cutting.

The conventional ceramic cutting tools shown as No. 26 to No. 29 inTable 1 have not been used for machining metallic materials called as"materials difficult to cut" such as chilled cast steel or forged steelsince the toughness values of these cutting tools are very low level.

Since an alumina base ceramic cutting tool is excellent in wearresistance but inferior in material strength, it can not be used for anapplication requiring tool toughness in the case of an interruptedcutting or machining of materials difficult to cut. A cemented carbidecutting tool is highly resistant to impact because of its high strengthand toughness, but tends to be worn in the early stage by the cuttingheat in machining. This is particularly remarkable, on high-speedcutting. Although a Si₃ N₄ cutting tool can perform a high-speed cuttingbecause it consists of ceramics containing no metal ingredient and hashigh impact resistance because of its high strength and high toughness,the adhesive and fall-off wear is going on by the reaction with the ironin the work material, and worn in the early stage, so that Si₃ N₄ is notsuitable for a material difficult to cut such as chilled cast steel.Further, for black ceramics and cemented carbide coating, the fall-offof grains appears as a wear or minute chipping in addition to breakingoff or chipping, and their lives were consequently shortened.

INDUSTRIAL APPLICABILITY

The alumina base ceramic sintered body according to this invention iswidely usable for members requiring toughness and wear resistance, andis sufficiently resistant under a severe condition of a high load at ahigh temperature, for example, as high-speed rough cutting of cast ironor steel.

    TABLE 1       -    Relative  Bending Fracture Wear Resistance Break-Off Chilled cast       Sample Composition (vol %) TiCN ratio Density Hardness Strength     Toughness Test Resistance Test Steel Cutting       No. WC TiCN W Ti MgO Y.sub.2 O.sub.3 Al.sub.2 O.sub.3 TiCTiN % HRA MPa     M Pam-1/2 Cutting time min FCD cutting Cuttable time: min       1 40 10 0.2    50 5:5 99.7 94.3 1002 3.9 18 ◯ 42       2 30 20  0.1   50 5:5 100.0 93.6 1000 3.9 16 ◯ 40       3 20 30 0.1    50 5:5 100.0 93.7 950 3.6 18 ◯ 41       4 10 40  0.2   50 5:5 100.0 93.8 950 3.7 15 ◯ 38       5 60 10 0.2    30 5:5 99.8 94.7 970 4.8 13 ⊚ 37              6 10 60  0.2   30 5:5 99.7 94.5 950 4.7 16 ◯ 33          7 40 40 0.2 0.2   20 5:5 99.6 94.5 920 4.2 12 ⊚ 35        8 35 35     30 5:5 99.7 94.4 880 4.1 14 ⊚ 37       9 30 30     40 5:5 99.8 94.3 850 4.0 15 ◯ 34       10 25 25     50 5:5 99.8 94.0 840 4.1 17 ◯ 35       11 20 20     60 5:5 99.9 93.9 800 3.7 19 ◯ 40       12 15 15     70 5:5 100.0 93.7 780 3.6 20 ◯ 38       13 30 30     40 2:8 99.8 94.3 860 4.1 15 ◯ 32       14 30 30     40 8:2 99.8 94.0 800 3.9 16 ◯ 35       15 30 30     40 0.2:9.8 99.5 93.4 680 3.5 14 ◯ Break-off     in 10 min       16 30 30     40 9.8:0.2 99.2 94.0 710 3.6 11 ◯ worn in 22     min       17 80 10     10 5:5 95.2 93.0 620 3.3 5 X Break-off in 5 min       18 60 0     40 5:5 99.6 94.5 930 4.3 13 ◯ worn in 22 min        19 40 0     60 5:5 99.7 94.2 900 3.8 11 ◯ worn in 26 min       20 10 80     10 5:5 94.0 92.8 590 3.1 3 X Break-off in 5 min       21 0 60     40 5:5 99.8 93.9 750 3.7 19 Δ Break-off in 22 min         22 0 40     60 5:5 99.9 93.8 720 3.6 21 Δ Break-off in 17 min       23 30 20   0.2  50 5:5 100.0 93.9 820 3.8 16 ◯ worn in 21     min       24 30 20    0.2 50 5:5 100.0 93.8 800 3.8 15 ◯ worn in 24     min       25 40 40     20 5:5 95.1 93.2 610 3.1 4 X Break-off in 5 min       26 Alumina (pure Al.sub.2 O.sub.3)  99.6 93.5 700 3.1 25 X Break-off     in 2 min       27 Black ceramics (Al.sub.2 O.sub.3 -30TiC)  99.9 94.0 800 3.5 21 X     Break-off in 7 min       28 Silicone nitride (Si.sub.3      N.sub.4)  98.9 93.5 1200 7.0 8 ⊚ worn in 15 min       29 Cemented carbide coating  100.0 91.0 230 10.0 12 ⊚     worn in 12 min              Cutting time up to              flank wear of 0.5              mm     ⊚ more than 0.7 mm/rev     ◯ 0.7-0.5 mm/rev     Δ 0.5-0.3 mm/rev     X: less than 0.3 mm/rev

What claimed is:
 1. An alumina base ceramic sintered body with highstrength and high toughness consisting of 5-70% by volume of WC, 5-70%by volume of Ti (C,N), and remainder of Al₂ O₃.
 2. An alumina baseceramic sintered body according to claim 1 consisting only three phasesof Al₂ O₃, VWC, and Ti (C,N).
 3. An alumina base ceramic sintered bodyaccording to claim 1 wherein the α-phase ratio in Al₂ O₃ crystal is 90%by volume or more.
 4. An alumina base ceramic sintered body according toclaim 1 and 2 wherein mole ratio of C:N in Ti (C,N) is within the rangeof 1:9 to 9:1.
 5. An alumina base ceramic sintered body according toclaim 1 wherein the average grain size of each of Al₂ O₃, WC and Ti(C,N) is 5 μm or less, respectively.
 6. An alumina base ceramicsaccording to any one of claims 1 to 5 which is used for a cutting toolor a wear resistant member.
 7. A method for producing an alumina baseceramics which comprises blending and sufficiently mixing powders of Al₂O₃, WC, Ti (C,N) having average particle sizes of 2 μm or less,respectively, so that the WC powder is 5-70% by volume, the Ti (C,N)powder is 5-70% by volume, and the remainder is Al₂ O₃ powder, sinteringthe resulting powder mixture by any one of ordinary temperaturesintering, hot pressing, and hot isostatic pressing, or the combinationthereof.